Such methods for ascertaining and monitoring fill level in a container are frequently applied in measuring devices of automation and process control technology. Such fill level measuring devices are produced and sold by the applicant, for example, under the marks, PROSONIC, LEVELFLEX and MICROPILOT. These devices work using the travel time measuring method and serve to determine and/or to monitor a fill level of a medium in a container. These fill level measuring devices transmit a periodic transmission signal in the microwave or ultrasonic range by means of a transmitting/receiving element in the direction of a surface of a fill substance and receive the reflected echo signals after a distance dependent travel time. Usual fill level measuring devices working with microwaves can be basically divided into two classes; a first class, in the case of which the microwaves are sent by means of an antenna toward the fill substance, reflected on the surface of the fill substance and then received back after a distance dependent travel time and a second class, in the case of which the microwaves are conveyed along a waveguide toward the fill substance, reflected on the surface of the fill substance due to the impedance jump existing there and the reflected waves led back along the waveguide.
Formed from the received echo signals, as a rule, is an echo function representing the echo amplitudes as a function of travel time, wherein each value of the echo function corresponds to the amplitude of an echo reflected at a certain separation from the transmitting element.
In this echo function, a wanted echo is determined, which corresponds to the reflection of the transmission signal on the surface of the fill substance. From the travel time of the wanted echo, there directly results in the case of known propagation velocity of the transmission signals, the separation between the surface of the fill substance and the transmitting element.
In order to simplify the echo curve evaluation, not the received, raw signal of the pulse sequences is used, but, instead, the envelope, the so called envelope curve, is ascertained. The envelope curve is won, for example, by rectifying the raw signal of the pulse sequences and then filtering via a lowpass filter.
There are a large number of different methods for determining the wanted echo in an envelope curve. These methods can be divided basically into two groups, the static detection methods with static echo search algorithms and the dynamic detection methods with dynamic echo search algorithms.
In a first method using a static echo search method, a static echo search algorithm is used to select as wanted echo that echo which has a greater amplitude than the remaining echos. Thus, the echo in the envelope curve with the greatest amplitude is designated the wanted echo.
In a second method using a static echo search method, a static echo search algorithm assumes that the wanted echo is the first echo occurring in the envelope curve after the transmission pulse. Thus, the first echo in the envelope curve is selected as wanted echo.
The two methods can be combined with one another into one static echo search algorithm e.g. by defining a so-called first echo factor. The first echo factor is a predetermined factor, by which an echo must exceed a certain amplitude, in order to be recognized as wanted echo. Alternatively, a travel time dependent echo threshold can be defined, which an echo must exceed, in order to be recognized as wanted echo.
In a third method, the fill-level measuring device is told the current fill level once. The fill-level measuring device can based on the specified fill level identify the associated echo as wanted echo and track such e.g. by a suitable, dynamic, echo search algorithm. Such methods are referred to as echo tracking. In such case, e.g. in each measuring cycle, maxima of the echo signal or the echo function are determined and based on knowledge of the fill level ascertained in the preceding measuring cycle and an application-specific maximum expected rate of change of the fill level, the wanted echo is ascertained. The new fill level then results from a travel time of the so ascertained, current wanted echo.
A fourth method is described in German Patent, DE 102 60 962 A1. There, the wanted echo is ascertained based on earlier data stored in a memory. In such case, from received echo signals, echo functions are derived, which reflect the amplitudes of the echo signals as a function of their travel time. The echo functions are stored in a table, wherein each column serves for accommodating one echo function. The echo functions are stored in the columns in a sequence, which corresponds to the fill levels associated with the respective echo functions. In operation, the wanted echos and the associated fill levels are determined based on the echo functions of the current transmission signals with the assistance the table.
Described in German Patent, DE 103 60 710 A1 is a fifth method, in the case of which periodically transmission signals are sent toward the fill substance, their echo signals recorded and converted into an echo function, at least one echo characteristic of the echo function is determined, and based on the echo characteristics of at least one preceding measuring a prediction for the echo characteristics expected in the case of the current measuring is derived. The echo characteristics of the current measuring are determined taking into consideration the prediction, and based on the echo characteristics the current fill level is ascertained. This method comes close to an echo tracking in the broadest sense.
German Publication, DE 10 2004 052 110 A1 describes a sixth method, which improves the wanted echo detection by an echo evaluation and classification of the echos in the envelope curve.
This above described methods work without problem in a large number of applications. Problems occur, however, when there is some doubt as to the correctness of the identification of the echo coming from the fill level.
In the case of the first method, for example, measurement problems occur when installed objects are present in the container, and the installed objects reflect the transmission signals better than the surface of the fill substance.
In the case of the echo tracking according to the third method, measurement problems occur, when during operation the wanted echo runs over a disturbance echo and subsequently the disturbance echo is tracked as a wrong wanted echo. Furthermore, a problem occurs, when upon turn-on, the preceding wanted echo signal no longer agrees with the actual situation or the preceding wanted echo signal is not known.
Summarizing, in the case of fill level measuring devices, which operate according to the travel time measuring principle, for ascertaining the fill level, the raw envelope curve is recorded and the echos arising from reflections on the surface of the fill substance and on disturbing installed items, such as, for example, tubes, stirring blade, filling flows of the fill substance, deposits, etc., are ascertained. For simplifying and improving the evaluation, the raw envelope curve is preprocessed, whereby a reduction of the maxima present and a quieting of the measuring can be achieved. Used for this preprocessing of the echo function, or the envelope curve, are filter methods, or statistical methods.
Application of the filter method is problematic, when different behaviors of the filters are required in different measuring situations. If random disturbance echos occur, strong filtering is necessary, in order to suppress them.
In the case of rapidly changing fill levels, this strong filtering causes, however, a significant problem in that virtual artifact echos, so-called “ghost echoes”, arise. These artifact echos can only be suppressed very difficultly and are interpreted by the evaluation software as possible fill-level echos. Due to this defective interpretation of an artifact echo as a fill-level echo, errors can arise in the fill level determination.
If mistakenly another echo, e.g. a disturbance echo, is classified as the fill-level echo, i.e. as wanted echo, of the fill substance in the container, there is the danger that a wrong fill level will be output. This can, depending on application, lead to overfilling of containers, to empty operation of pumps or other events possibly associated with considerable danger.